Optical pointing devices are already known in the art. U.S. Pat. No. 6,806,458, filed in the name of the same Assignee and which is incorporated in its entirety herein by way of reference, for instance discloses a method, a sensing device as well as an optical pointing device including a sensing device for comparing light intensity between pixels.
U.S. patent application Ser. No. 11/011,164 filed in the name of the same Assignee and published as the US Patent Application Publication No. 2006/0125794, which is incorporated in its entirety herein by way of reference, also discloses a method for detecting a lift condition from an illuminated surface portion of an optical motion sensing device. Providing either two first “loss-of-focus” thresholds, the use of which depends on whether the optical pointing device is moving or not, or a dynamical loss-of-focus threshold depending on an average number of motion features that the surface exhibits to the sensor of the optical pointing device.
However optical pointing device such as mouse using LED as illumination source does not present the expected tracking performances for all kind of surfaces, in particular on wood. Using instead a laser illumination source results in a speckled image detected by the photodetector of the mouse sensor. Then a key problem with this speckled image is that it has a high spatial frequency and is no longer well adapted to the previous methods of “Lift Detection” described in the prior art.
Actually, it is difficult to correctly determine when the mouse is lifted, how to remain in a lift condition, and how to detect when the mouse has been returned to the surface or dropped. As previously discussed, the lift case can be considered as a “loss of focus”; the focal depth of the imaging lens being short, when the mouse is lifted, the image on the sensor will become defocused. When such a “loss of focus” occurs, the optical power on the sensor should drop dramatically. In order to identify a “loss of focus”, the drop in optical power could be sensed. One way to do it is to monitor the “Integration Time”. The “Integration Time” is the time that the pixel sensor takes to reach a determined target intensity level. When this integration time rapidly increases, then a “Loss of Focus” can be declared. However, these conditions to detect loss of focus are not always sufficient to reliably detect lift. The sensor must properly handle lift mode and avoid detecting false drops to optimize its power consumption.
There are further complications to accurately detect lift. The detection of lift via a drop in optical power is dependent on the accuracy of the sensor and the processing electronics as well as the illumination source. It has been seen that the power does not always drop as expected and that lift is not detected at an acceptably low elevation.